Signal transmission system



G. L. FREDENDALL. 2,330,965

Oct. 5, 1943.

SIGNAL TRANSMISSION SYSTEM Filed March 12, 1941 2 Sheets-Sheet l HG. j. PII/96E CUNTFOL NETWORK /j /7 /3 'rELEv/s/o/v EMPL/WE1? dig/@HH WND Ewan/MEE MDUL/W'OF PHE-EMPL/E/EH lf-Jn) Jn 'A 7:5

df I df vx EHEuE/vcy the reproduced picture.

Patented Oct. 5, 1943 UNITED STATES PATENT vOFFICE SIGNAL TRANSMISSION SYSTEM Gordon L. Fredendall, Westmont, N. J.,y assignor to Radio Corporation of America, a corporation of Delaware Application March 12, 1941, Serial No. 382,880

(Cl. P18-7.1)

6 Claims.

This invention relates to signal transmission systems and more particularly to a method and means for correcting phase distortion in signal transmission systems employing carrier modulation.

It is well recognized that in'wide band amplifiers, such as those utilized in television, in contradistinction to amplifying systems for sound, the phase relation between input and output of the various frequency components present in the video signals must be rigorously maintained because any relative phase displacement between these components results in a loss of definition in exceedingly important that the low frequency components have their initial phase relation with respect to the higher frequencyy components free from `any variation or deviation, since the low frequency components represent the background values, while the highvfrequency components determine the sharpness `of resolution of the reproduced image.` Any phase displacement between thesev two sets of components destroys the sharpness of detail inasmuch as the highvfrequency components do not coincide in proper spacial relation with the background so that instead of a In this connection, it is,

Figures 3a, 3b and 3c are explanatory diagrams relating to the operation of this invention,

Figure 4 is agraphical illustration lshowing the phase delay characteristics of a normal transmission system,

Figure 5 is a graphical illustration of the phase characteristic of an auxiliary circuit for the correction of the distortion shown in Figure 4,

Figure 6 is a normal desired amplitude response characteristic for a vestigial side band television transmission system, y

Figure 7 is a circuit diagram of a band pass filter having a phase distortion which may be compensated for by the use of this invention,

Figure 8 is a graphical illustration of the amplitude and phase delay characteristics of a typical television signal transmission system, and

Figure 8a is the desired phase delay compensation characteristic curve necessary to correct the 'phase delay characteristic shown in Fig. 8.

Figure 2 showsa modulating signal e(t) such as that of curve a-b, which may be regarded as sharp and clear image a distorted image is reproduced.

Phase distortion is likely to result from the use of the broad band ampliersvin the carrier modacteristic is complementary to the phase distor-` tion characteristic of the modulated carried portion of the transmission system.

'I'he primary object of this invention is to im- I prove the transmission characteristics of broad frequency band amplifiers.

Other objects 'of this invention are to improve the phase response characteristic of transmission systems and to correct for the normal phase dis-k tortion present in a television signal transmission system.

Other and incidental objects of this invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawings, in which Figure 1 is a combined block and schematic diagram showing one form of this invention,

Figure la is a graphical illustration of one possible phase delay vcharacteristic afforded by constant resistance lattice filters,

Figure 2 is a graphical illustration of a modulating signal,

a periodic` function if the proper fundamental period is chosen. That is,

Where e(t) is the modulation signal n is the order of the harmonic An is the'amplitude of the 'nth harmonic fo is the fundamental frequency 0n is the phase angle of the nih harmonic is the conventional Fourier representation of a periodic wave.

If a carrier wave sin 21rfct is modulated by (l), the side bands take the form` 2 cos [2me-nauwe and where m is the modulation factor, fc is the carrier frequency, t is time.

If the carrier` frequency circuits preceding the point of detection or reception of the envelope introduce a phase shift, 2) becomes ,9n-` and n+ refer to the phase shifts associated with the lower and upper side bands respectively. faand n+ will be regarded as parts of a continuous curve Unless is nearly a straight line throughout the essential frequency band, the' When nand n+ are related in a special sense and an suitably chosen such that constant the Wave shape of the envelope will not be subject to distortion due to non-linearity of Fig. 8b shows the special relation that must n exist between the two branches of i. e. ,8nand c+. The special relation is that the branches are symmetrical about' the point representing the frequency of the carrier fc.

Fig. 3a shows the variation of la df which satisfies Equation 5.

Fig. 3c represents the curve which results from adding the derivative curves to Figs. 3b and 3a.

A particular application of this invention is in a television system. An easily obtained overall delay characteristic of the. r-f circuits of a television transmitter and receiver and the i-f circuits of a receiver is shown by df in Fig. 4.

i@ df may include -as many of the carrier circuits as are desired to be compensated.A

df in Fig. 5 represents the delay of compensating circuits inserted in the modulating (video) circuits of the transmitter. It is seen that d??? and l.-

have equal but opposite curvatures. The overall amplitude characteristic of the carrier circuits is shown in Fig. 6. It is seen that the essential frequency spectrum extends from f1 to fz. Hence the variations of and beyond these limits are unimportant. For the purpose of illustration, let it be assumed that a simple band pass filter is used in the television signal carrier frequency circuit.

Fig. 7 shows a simple band pass filter of the type used in the intermediate frequency amplifier of a television receiver and having a phase For purposes of the example let f1=9.25 mc.

f2=12.75 mc.

R need not vbe specified.

Fig. 8 illustrates the amplitude and delay characteristics of such a circuit. It will be noted that at the carrier frequency the amplitude response tapers off to substantially zero at a frequency in megacycles of approximately 14.25'. In the region adjacent the carrier frequency the phase delay characteristics become of such magnitude as to cause serious trouble in the reproduction of the television image. The rangepf frequencies with which we are particularly concerned are those frequencies on the phase delay characteristic curve indicated by the portion of the curve Let it be assumed that the carrier wave fu located at 12.6 mc., which defines the line A-B, a line about which the portion b--c-d of the vdelay characteristic is reasonably symmetrical. The shape of the arc d-e corresponds t'o a region of the amplitude characteristic which is less important because the amplitude of the signal in this-range is relatively small. The delay characteristic of the correcting phase control network must be complementary to b-c-d as shown by crve d'' in Fig. 8a.

Fig. 1 illustrates by block I a television camera and preamplifier which may be of the type, for example, as shown by Iams and Rose in an article entitled Television pickup tubes with cath-s ode rayy beam scanning, in the Proceedings of the I. R. E, for August, 1937. The preamplifier included with the television camera. I may be of the type such as described in an article entitled "Analysis and design of video amplifiers," by S. W. Seeley and C. N. Kimball, pages 241 to 255, in Television, vol. 2, published by the RCA Institutes Technical Press, New York. Y

A phase control network includes a plurality of sections employing the Well known principles of filter design, such as described by Otto Zobel in an article entitled Distortion correction in electrical circuits with constant-,resistance recurrent networ pages 438-534, Bell System Technical Journal, July, 1928, vol. 7, No. 3. For example, each section of the phase control network includes capacities 3 and inductances 5. By choosing the correct number of sections and by applying the proper components of each section in accordance with the instruction given by Zobel in the above identified article, it is possible to compensate in the modulating signal channel for any phase error characteristic of the carrier modulated signal channel.

For example, the observed phase characteristics illustrated in Fig. la are approximated by choosing the circuit contacts of filter sections of the type shown in Fig, 1 such that the phase delay of individual sections has the character illustrated by n or n in Fig. 1a. 4 The different frel quencies of maximum delay doo f for the various sections are chosen to give the best overall approximation to the desired phase delay curve. y and 1, are adjustable constants, the values of which determine the circuit constants of a particular phase correcting filter sections.

The output signal from the phase control network is applied to an amplifier modulator 1 to modulate transmitter 9 of'any suitable type.

The scope of this invention is not limited to those particular modifications set forth above but it also includes partial phase delay compenv sation carrier frequency circuits having overall delays which do not meet the requirement of symmetry but which may be decomposed into components, some of which are symmetrical over the essential frequency band. The symmetrical components are compensated in the modulating circuits of the transmitter, according to the principles disclosed above.

While several systems for carrying this invention into effect have been indicated and described, it will be apparent to one skilled in the art that this invention is by no means limited to the particular organizations shown and described. but that many modifications may be made without departing from the scope of this invention as set forth in the appended claims.

What is claimed is:

1. In combination, a signal carrier transmission system, a modulating signal channel, and means in said channel for compensating phase distortion in said carrier transmission system comprising a network having a phase characteristic complementary to the phase characteristic of said carrier transmission system.

2. In a television system, a signal carrier transmission system whose uniform phase characteristic response is sacriced for an improved amplitude response characteristic, and a modulating signal channel including a network having a phase characteristic complementary to the phase characteristic of said carrier transmission system.

3. In a television system of the type wherein a television transmitter supplies modulated carrier signals for a plurality of receivers whose overall phase response characteristic is nonuniform over the desired frequency pass band in said transmitter, a modulating signal channel whose phase response characteristic is complementary to the phase response characteristic of said receivers.

4. In a television system of the type wherein a television transmitter having a modulating signal channel supplies signals for a plurality of receivers each including an intermediate frequency amplifier having a phase response characteristic nonuniform over its useful pass band, means in said modulating signal channel for causing said modulating signal channel to have an overall phase response characteristic complementary to the phase response characteristic of said intermediate frequency amplifier.

' 5. In television systems of the type embodying a modulated carrier channel and a modulating signal channel, the method of reducing phase distortion including the step of compensating in said modulating signal channel for the phase distortion caused in said modulated carrier channel.

6. The method of reducing distortion in the reproduction of television images resulting from the nonuniform time delay of different frequency components of the modulated carrier circuits in a television system comprising the step of com.- pensating for said distortion in the modulating signal channel of said system.

GORDON L. FREDENDAIL. 

